With the release of HPC Pack 2012, the HPC Pack 2008 R2 Enterprise version is now available for customers who are not able to run Windows Server 2012 on their head node. When running 'setup' you may select either the 'Express' or 'Enterprise' option without needing an additional license.
You must still have a licensed version of the Windows Server 2008 R2 Standard, Enterprise, Datacenter or HPC Edition operating system to run it. The Windows Server 2008 R2 HPC Edition operating system is no longer available.

Download the Microsoft HPC Pack 2008 R2 '.zip' archive from the link in the 'Files in This Download' section. Extract the files to the root of your drive and run 'setup.exe' to start the HPC Pack installation wizard on a supported Windows Server 2008 R2-based computer. On the 'Select Installation Edition' screen, select the 'HPC Pack 2008 R2 Enterprise' option.